I run XMBC,but have the same blank screen issue.
Joining the party towards the end. The blank screen issue seems to stem from an issue with DirectX. If an application, say WMC or XBMC, is running full screen when the display is turned off for an extended period of time (overnight) then the application needs to be taken out of full screen mode to get the blank screen to go back to the desktop. To help resolve this someone in another forum posted an event ghost macro to detect the display on/off trigger. It then toggles the fullscreen option in xbmc. When the display kicks back on, the application full screen toggle goes back to full screen. A little googling should point you in the right direction.
On the harmony remote you can add the "Windows Media Center SE" keyboard as a device. It contains the key combinations like alt+F4 and a few other commonly used key combinations.
Device type: Computer -> Media Center PC
Model: Windows Media Center SE
If you set your media center to run on a shortcut key, it's real easy to add the kill/restart macro on the harmony. My harmony macro is something like this:
XBMC (F10), wait 2 seconds, alt+F4, wait 2 seconds, restart xbmc (F10).
Best of luck, we're all in this together.